clang -O2  -Wall -Wextra -Werror=implicit-function-declaration unistd/setpgid.c -o unistd/setpgid -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
unistd/setpgid.c:18:8: error: implicit declaration of function 'getpgid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
                if ( getpgid(0) != getpid() )
                     ^
1 error generated.